1. Field of the Invention
The present invention relates to an optical scanner used in an image reproducing apparatus, such as a digital photocopy machine or a laser printer, and to an image reproducing apparatus using such an optical scanner.
2. Description of the Related Art
To increase the image reproduction speed in an image reproducing apparatuses, such as digital photocopy machines or laser printers, an optical scanner that has a common light deflector arranged between pairs of scanned planes in order to guide the beams onto the scanned planes located on both sides of the common deflector has been conventionally known. With such an optical scanner, optical scanning is carried out along the multiple scanned planes simultaneously. For instance, exposure can be performed simultaneously for four colors (yellow, magenta, cyan and black), and consequently, a color image can be reproduced quickly.
Another type of known optical scanner increases the rotating rate of the light deflector to increase the scanning rate on the scanned plane in order to realize high-speed image reproduction.
Still another type of known optical scanner uses a light deflector (e.g., a polygon mirror) having an increased number of facets (as deflecting reflection planes) to increase the image reproduction speed, without changing the rotating rate.
When using the optical scanner with the scanned planes arranged on both sides of the optical deflector, the light beams used to illuminate the scanned planes have to be received at beam detectors prior to the optical writing for the purpose of synchronization of writing start timing. If the light deflector is shared between a pair of optical scanning systems, the scanned planes located on both sides of the light deflector are optically scanned by the beams in directions opposite to each other with the writing start positions inverted. For this reason, the beam detector of one optical scanning system has an inverse arrangement with respect to that of the other optical scanning system located on the opposite side of the light deflector. This arrangement makes it difficult to maintain the required scan width unless the area size of each reflection facet of the light deflector is increased. However, when the reflection facet is made larger, a large-sized motor is required to rotate the light deflector. This causes noise, vibration, power consumption, and cost to be increased.
The same problem occurs in the arrangement of increasing the rotation rate of the light deflector, causing noise, vibration, power consumption, and cost to be increased.
In the arrangement for increasing the number of facets of the light deflector, each reflecting surface of the light deflector has to be made large in order to guarantee the necessary scan width. This also causes the above-described problems, that is, increased noise, vibration, power consumption, and cost. In addition, since the distance from the rotational center to each reflecting surface becomes long, the sag (difference in position of reflection point depending on angle of view) becomes large, and consequently, the optical characteristics (such as curvature of field and linearity) are degraded.
Some techniques for increasing the image reproduction rate without increasing the revolution rate of the light deflector have been proposed, which are all applied to the optical scanner with the scanned planes arranged on both sides of a common light deflector.
For example, JPA 9-58053 discloses a technique for correcting color misalignment among multiple scanned planes. In this technique, multiple beams are produced, and each beam is detected on the writing starting side and the writing ending side using beam detectors. The clock frequency of each beam is adjusted so as to correct the color misalignment.
Another publication, JPA 9-127443, proposes to set the incident angles of the beams incident on a common light deflector differing from each.
Still another publication, JPA 2001-350110, proposes to arrange a pair of optical scanning systems asymmetrically with the incident positions thereof offset from each other. This arrangement aims to maintain the effective writing range in common between multiple scanned planes, while providing beam detection areas for placing the light-receiving means, without increasing the size of the deflecting/reflecting surface of the light deflector.
However, the technique disclosed in JPA 9-58053 requires spaces for placing the light receiving means (or beam detectors) near the writing start position and the writing end position. Accordingly, it becomes difficult for this technique to provide the required scanning width.
With the technique disclosed in JPA 9-127433, the lens systems before the deflector have to be arranged differently from each other in order to make the incident angles of the beams onto the deflector differing from each other. Accordingly, the layout of the optical scanning systems becomes difficult.
With the technique disclosed in JPA 2001-127443, because the optical scanning systems are arranged asymmetrically on the right side and the left side of the light deflector, with the incident positions differing from each other, dot positions are offset in the fast scan direction.